From The RadioReference Wiki
Cell blocked (or cellular blocked) is a phrase applied to scanners and wideband receivers manufactured for sale in the US which denotes that they comply with the provisions of PL 102-556, which amended Section 302 of the Communications Act (47USC302) to prohibit manufacture, importation, or certification of scanners which could receive the frequency band allocated for analog AMPS cellular telephony, "the frequencies allocated to the domestic cellular radio telecommunications service":
The ban came to be when an amendment was put into the bill in question, S.1579-1991, on 10/7/1992 by Sen Larry Pressler of SD; the bill deals mostly with procedures for pay-per-call telephone services.
The motivation for this amendment had to do with an analog AMPS cellular telephone call into a political conference call made from Florida by now-incoming Speaker of the House John Boehner in which there was discussion of the ethics charges against then-speaker Newt Gingrich; a recording was made of the call and given to a Democratic Florida politician and whackiness ensued.
Present Day Impact
The language of the bill does not cover the frequencies used by iDEN and PCS services, which have always been digital, but it now covers frequencies which are only used by encrypted digital services, which standard receivers cannot decode.
The block is therefore no longer useful--since all currently in-use US cellular protocols are digital and encrypted--but required to be there anyway, since that section of the law was never repealed.
[ Thanks to Thomas.loc.gov, even though I can't reasonably cite anything because 14 years later, their search engine doesn't return actual URLs, and also to Wikipedia for the actual freqs, and our own NF2G for the source material which led me to the law. ]